Wide-field-of-view projection display

ABSTRACT

A wide-field-of-view projection display comprises a circularly symmetric lens and an array of light emitters, positioned along the focal circumference of the circularly symmetric lens so that light from each of the light emitters is substantially collimated by the lens in a different direction. A ray-diverting means, such as a slab waveguide or a reflector, ejects the collimated light out of the plane of the lens to the viewer. The planar circularly symmetric lens has no aberration, allowing adjacent views to be seamlessly joined because they can all be diffused by the same angular amount.

This invention relates to the field of 3D displays, head-mounteddisplays and moving projection displays and is a way of increasing theirfield of view.

Projection displays conventionally comprise a two-dimensional (2D) arrayof light emitters and a projection lens. The lens forms an image of thearray at some plane in space, and if this imaging plane is far from theprojection lens then the effect of the projection lens is to collimatelight from any pixel on the two-dimensional array.

It is also possible for a large-diameter projection display to be placedbehind a liquid-crystal a display or some other spatial light modulatorin order to synthesize a three-dimensional image, as described inTravis, A. R. L. , “Autostereoscopic 3-D Display,” Applied Optics, Vol.29, No. 29, pp4341 to 4343, Oct. 10, 1990. One pixel at a time of thetwo-dimensional array of light emitters is illuminated, and anappropriate view of a three-dimensional object is thus simultaneouslydisplayed on the liquid-crystal display in such a way that the view ofthe three-dimensional object is only visible if observed from thedirection in which the rays of light collimated by the projection lensfrom the pixel are travelling. A sequence of views is repeated at a ratefaster than that at which the eye can detect flicker, therebytime-multiplexing a three-dimensional image. It is furthermore possiblein principle to create a holographic three-dimensional image by placinga two-dimensional array of point source light emitters in the focalplane of the projection lens, illuminating each point source in turn,and displaying appropriate holograms on a liquid-crystal display placedon top of the projection lens so that each hologram is made visible to adifferent point of view in turn.

Projection displays are most commonly directed so that the image of thearray falls on a large translucent screen, and a viewer looking at thescreen will see a greatly magnified image of the picture that isdisplayed on the two-dimensional array. However, it is becomingincreasingly common for small projection displays to be mounted on thehead of the viewer so that the projection display is directed towardsthe viewer's eye, and light collimated by the projection lens from asingle pixel on the two-dimensional array of light emitters issubsequently focused by the viewer's cornea onto the viewer's retina sothat the viewer sees an apparently distant image often known as avirtual image.

Head-mounted displays are bulky and users would prefer them to be flat.A head-mounted display can be made flatter, for example, using a slabwaveguide incorporating a weak hologram, as disclosed in Amitai, Y.,Reinhorn, S. and Friesem, A. A., “Visor-display design based on planarholographic optics,” Applied Optics, Vol. 34, No. 8, pp. 1352 to 1356,Mar. 10, 1999. Light from a cathode-ray tube and a further hologram canbe coupled into the waveguide, and this light will be diffracted out ofthe waveguide by the weak hologram in directions which are determined bythe pixel within the cathode-ray tube from which the light was emitted.

Three-dimensional images synthesized as described above bytime-multiplexing the illumination of a liquid-crystal display requirethe liquid-crystal display to have a fast-switching array of thin-filmtransistors and these are expensive. Trayner and Orr (U.S. Pat. No.5,600,454) have demonstrated a device which avoids this by placing ahologram behind a conventional liquid-crystal display that directs theillumination of alternate rows to a left-eye or right-eye view. But boththis and the switched illumination concept described above are bulky,and users would prefer that three-dimensional displays were flat.

Instead, a flat-panel three-dimensional display can be made by combininga projection display with a screen on which light shone parallel to thesurface of the screen is ejected at one of a set of selectable linesalong the screen, as disclosed in WO 98/15128. One line at a time on thescreen is selected, and simultaneously the projection display projects aline of pixels parallel to the screen so that they are ejected at theselected line. The same line of pixels on the projection display isaltered repeatedly as each of the series of lines on the screen isselected in turn in such a way as to time-multiplex a complete image onthe screen only one line of the projection display is used, so the arrayof light emitters need be only one line high, and if the emitted lightis collimated in the plane of the screen then the projection lens needbe only one or two millimetres high so that the combined projector andscreen are flat.

With this construction if it is light from a three-dimensional display,albeit one whose array of light emitters is only one pixel high, whichis directed parallel to the surface of the screen of selectable lines,then the image formed on the screen is three-dimensional. Thethree-dimensional display might comprise an array of light emittersbehind a projection lens with a liquid-crystal display in front of theprojection lens as described above, but in order to put up several viewswithin one line period of the display, the switching rate of the liquidcrystals would need to equal the number of views times the line rate ofthe display, and few liquid-crystal mixtures switch this fast.

Many other kinds of autostereoscopic and holographic three-dimensionaldisplay concepts exist and any could possibly be adapted to be used in aflat-panel system. Particularly interesting is an old concept comprisinga group of small video projectors in the focal plane of a field lens—seeA. R. L. Travis, Proc. IEEE Vol. 85, no. Nov. 11, 1997, pp. 1817-1832.Each projector is positioned to form a view in the plane of the fieldlens just as if the lens were a translucent screen, but unlike atranslucent screen the field lens collimates the light so that thepicture is visible from only a single direction. The other projectorsform views which are made visible by the field lens to other directionsso that the viewer sees an autostereoscopic three-dimensional image.

The problem with this concept is that it is difficult to design aprojection lens whose pupil equals the lens's physical diameter; as aresult there are gaps between the video projectors which form dark zonesbetween adjacent views of the three-dimensional image. A slightlydiffusive element can be used to reduce these gaps, but the angle ofdiffusion usually varies with incident light angle. Aberrations in thefield lens mean that rays collimated by the lens from a single point hitthe diffusion screen at slightly different angles of incidence over thediameter of the screen. This means that the angles of diffusion vary,and even though the variance is slight it is enough to cause observablegaps between the views nearer-normal (if the projector spacing is set toeliminate all overlap) or overlaps between views (if the projectorspacing is set to eliminate gaps).

In fact, a further major problem with three-dimensional displays andhead-mounted displays in particular is that their field of view islimited by the aberration of the projection lens. At fields of viewbeyond 20° the lens collimates light so poorly that the image is toodistorted for most applications.

The present invention aims to overcome or at least alleviate some of theproblems with projection displays known in the art.

The present invention contemplates a wide-field-of-view projectiondisplay making use of a circularly symmetric lens, sometimes called amonocentric lens, and a corresponding curved array of light emitters;the centre of curvature of the array is at the centre of the circularlysymmetric lens and the array is placed at or near the focal plane of thecircularly symmetric lens. Circularly symmetric lenses have been usedbefore—see U.S. Pat. No. 5,132,839 (Travis), but they are difficult tomanufacture in a bulk-optic design.

According to a first aspect of the invention therefore there is provideda wide field of view projection display comprising: a generally planarcircularly symmetric lens and an array of light emitters, positionedalong the focal circumference of the circularly symmetric lens so thatlight rays from each of the light emitters is substantially collimatedby the lens in a different direction from its neighbouring lightemitters; and a ray-diverting means for ejecting the collimated lightout of the plane of the lens towards the viewer.

The ray-diverting means is generally in the shape of a panel co-planarwith or parallel to the plane of the lens and preferably comprisesline-selecting means for selecting one line at a time of a changingimage from the array of light emitters so as to display that line. Inthis case, the ejection may be brought about by deflection of the raysfrom the panel at the selected line. For example, the panel may includea reflective sheet and a transducer for producing a localized, linear,acoustic or surface wave in the sheet, the presence of the wave at agiven position causing reflection of the ray and thus the saiddeflection at that position. In this construction, the wave created bythe transducer travels along the panel successively ejecting the linesin its path. Alternatively a rotating-mirror arrangement can be used toscan each line over the height of a screen.

Alternatively the diverting means may be a flat panel of material whichis not opaque to the rays and which is preferably parallel to the lensand aligned with the lens edge through which the rays emerge, so that itacts as a slab waveguide.

The line-selecting means may be provided by a layer or strip on thepanel or at any other position in the collimated beam of light, which isswitchably reflective or transparent, the means for selecting theposition at which the rays are ejected being adapted to change the stateof the switchable layer. Such a layer may be a liquid-crystal display.

The switchable layer may work in transmission (so that the rays in theselected line travel through the layer and others are reflected) or inreflection (the selected line only is reflected. In this latter mode,the grating may be arranged to eject light only out of one surface ofthe panel in the direction of the switchable layer. Such a grating maybe positioned within the panel. The selected line is then reflected backthrough the panel by the switchable layer. In this mode, the selectingmeans are therefore provided behind the panel, from the viewer'sperspective. Reference may be made to WO 95/15128, mentioned above, formore information.

Each light emitter used in the present invention may include amicrodisplay, generally a small LCD with a collimated light sourcebehind it, if transmissive, or in front of it, if reflective. The lightemitter may simply be a microdisplay positioned at the focalcircumference. Alternatively, each light emitter may comprise amicrodisplay and an individual lens, arranged so that the microdisplayemits light which is then converged towards the individual lens. Eachindividual lens should be positioned on the focal circumference of thecircularly symmetric lens and acts essentially as a point source fromwhich collimated light can be produced.

In general the microdisplay will be one dimensional, consisting, of arow of columnar pixels, and the corresponding lens is cylindrical andseparated from the microdisplay by its focal distance if the lens iscylindrical, the microdisplay is thus positioned on the focalcircumference. Neighbouring microdisplays each project a single-lineimage of an object, the images differing only in the angle of view.

Alternatively, each light emitter may simply comprise a source of lightpositioned on the focal circumference of the circularly symmetric lens.In this construction, a one-dimensional switchable strip is provided inthe path of the collimated rays. The strip is preferably between thecircularly symmetric lens and the ray-diverting panel. If the lightsources are point sources, the strip may be used to display a hologramby suitable addressing of the strip. Alternatively, abutting sources canbe used to display an auto-stereoscopic view. The emitters are activatedin turn and the pattern displayed on the strip is synchronized with theactuation of the emitters.

The display according to the invention may include a diffuser positionedin the collimated light to narrow gaps in the beam between the lightfrom each light emitter. The diffuser may be formed as a diffractiongrating or lenslet screen and is preferably positioned adjacent to theflat panel. The diffuser will generally be necessary forautostereoscopic displays in order to ensure that there are no gapsbetween views; holographic displays will not generally need a diffuser.

A frame store may be provided for each microdisplay to store successiveimages of a moving display before they are: applied to the projectors;this makes it possible to compensate for any optical deficiencies, orgeometrical distortion such as shear due to the angle of projection ofthe side projectors.

In one preferred embodiment of the invention, a reflector, such as amirror, is provided to at least one side of the panel to reflect anouter portion of the images from the more off-axis projectors that missthe panel, back onto the panel. Such a reflector reduces the gap thatmay be produced at the side of the image by rays from outermicrodisplays in the array. A straightforward reflector would reflectthe outer image portion across to the opposite of the image from itscorrect position. Preferably therefore, image-processing means areprovided, to ensure that the reflected, pixels are reflected onto thecorrect side of the image. These may be provided in conjunction with theframe store and act to swap pixels at the outer edges of the framestore.

The display according to the invention may be arranged with thecircularly symmetric lens, panel and microdisplay (where applicable) insubstantially the same plane. Alternatively, the planes in which thepanel and lens are formed may be adjacent and parallel. In this case,folding means are required to fold the optical system so that raysemitted from the edge of the lens are directed onto the panel. Thefolding means may also fulfill the function of retrieving rays whichwill otherwise miss the panel. The folding means may comprise aretroreflector, preferably situated next to the portion of the lens fromwhich the rays emerge, and angled mirrors to either side of theretroreflector. The retroreflector is preferably positioned in a planesubstantially perpendicular to the side mirrors and the prisms of theretroreflector run perpendicular to its longitudinal axis.

In another embodiment, for a virtual display, the rays at any positionon the panel are ejected. To this end, the panel may include a weakdiffraction grating, which causes collimated light to travel in aparticular direction. The grating should be provided on the side of thepanel through which the rays are ejected.

Specific embodiments of the invention will now be described by way ofexample with reference to the accompanying drawings, in which:

FIG. 1 illustrates the use of a monocentric lens in accordance with theunderlying principle of the invention;

FIG. 2 illustrates a wide-field-of-view multiple-projectorautostereoscopic three-dimensional display according to a firstembodiment of the invention, where the screen is only one pixel widthhigh;

FIG. 3 illustrates a flat-panel wide-field-of-view autostereoscopicthree-dimensional display making use of the embodiment of FIG. 2,showing a screen on which light shone parallel to the surface of thescreen is ejected at one of a set of selectable lines along the screen;

FIG. 4 illustrates how a diffuser is used to eliminate the gaps betweenadjacent views on a wide-field-of-view three-dimensional displayaccording to the first embodiment;

FIG. 5 illustrates how, without anticipatory correction, rays from anoff-axis microdisplay on a wide-field-of-view three-dimensional displayaccording to the first embodiment will produce a view distorted byshear;

FIGS. 6a and 6 b illustrate how a pair of mirrors fold rays fromopposite off-axis microdisplays so that rays folded from onemicrodisplay fill in the gaps of a view left by rays from the oppositemicrodisplay;

FIG. 7 illustrates a wide-field-of-view flat-panel projection displayaccording to an embodiment of the invention, using a diffractiongrating;

FIG. 8 illustrates a holographic wide-field-of-view flat panel displayaccording to this embodiment, using a liquid-crystal panel;

FIG. 9 illustrates the display according to a development of the secondembodiment and shows how a pair of mirrors and a one-dimensionalretroreflector can keep illumination uniform even at large off-axisangles;

FIG. 10 illustrates a wide-field-of-view three-dimensional flat paneldisplay using a light valve, representing a third embodiment;

FIG. 11 illustrates a holographic wide field-of-view flat-panel displaywhich needs no thin-film transistors;

FIG. 12 illustrates a wide-field-of-view flat-panel head-mounteddisplay;

FIG. 13 shows a fourth embodiment of the invention; and

FIG. 14 is a sectional view of a lens and grating combination similar tothat shown in FIG. 10.

Referring to the drawings, the projection display of FIG. 1 comprises acircularly symmetric lens 1 and an array of light emitters 2 which iscurved so that each of the light emitters 2 lies in the focal plane orfocal surface, of the circularly symmetric lens 1.

The circularly symmetric lens 1 comprises a series of concentriccoplanar transparent annuli whose refractive indices are chosen so thatthe lens 1 collimates light from any point on the focal plane. Betweenthe edges of adjacent annuli a conventional anti-reflection coating,maybe applied. For example, a disc of polymethyl methacrylate of radius 50mm inside an annulus of polycarbonate of internal radius 50 mm andexternal radius 100 mm will collimate light from any point on a focalplane or ring of radius 172 mm. Alternatively the circularly symmetriclens 1 may comprise a graded-index disc whose refractive index varieswith radius and is largest at the centre. A second alternative is thatthe circularly symmetric lens 1 may comprise a disc of material whosethickness varies with radius. In this case, light from the array oflight emitters 2 is injected into the edge of the disc at a singleangle, slightly off, the normal to the disc axis; the flat surfaces ofthe disc make it behave like a slab waveguide and guide light by totalinternal reflection from one edge to the other. The disc becomes thickertowards the centre, and as rays are guided into thicker parts of thedisc the angle between ray direction and the axis of the disc becomessmaller. The resolved part of the ray direction in the plane of the disctherefore becomes smaller, so that the rays take longer to travelthrough the centre of the disc than the edges. The disc thereforecollimates light in much the same way as a graded-index lens.

In the autostereoscopic three-dimensional display of FIG. 2, each lightemitter in the curved array of light emitters 2 comprises a smallcylindrical lens 3 illuminated by collimated light from a laser source(not shown), reflected off a microdisplay 4 and converged by a furtherlens or lenses, likewise not shown, onto the small lens 3 which thusacts as a small light source. The microxdisplay 4 could instead betransmissive. Each microdisplay consists of a row of vertical pixels.

In FIG. 3, light emitted from the small lens 3 after modulation by themicrodisplay 4 is converted to a parallel beam by the circularlysymmetric lens 1 and shone parallel to and slightly above the surface ofa sheet of reflective foil 5. A transducer 6 at one end of the sheet offoil 5 sets up a single surface wave 7 which travels the length of thefoil 5 in the direction of propagation of the light, and which reflectsthe injected light at different lines along the foil S as the surfacewave 7 travels, because its height is sufficient to intercept the lightfrom the lens 1.

Arrangements of this sort are described in the inventorys earlier WO96115128.

If we consider light from a single microdisplay 4 at a single instant,the light will be modulated by the microdisplay 4, converged on to andthen expanded by the small lens 3 and collimated by the circularlysymmetric lens 1 to produce a series of parallel rays travellingparallel to the plane of the foil 5. When this light hits the surfacewave 7 the light will be ejected from the screen in a particulardirection within the horizontal plane containing the wave (taking thefoil to be vertical), and if a viewer observes the foil 5 from thatdirection he will see a line of pixels visible at the surface wave 7. Insuccessive instants as the surface wave 7 moves down the sheet of foil5, lines of pixels can be made visible at other positions on the sheetof foil 5, and if this is repeated sufficiently quickly the viewer willsee a time-multiplexed two-dimensional image.

This idea can be taken further by including further microdisplays atdifferent angles around the axis of the lens, to increase the range ofpossible angles of view. In a similar manner these other microdisplays 4can be modulated to produce other two-dimensional images on the sheet offoil 5, but each two-dimensional image will be visible from a differentdirection in the horizontal plane, i.e. in azimuth. If eachtwo-dimensional image is a view of what the viewer would see were thereto be a three-dimensional object in place of the sheet of foil 5, thenthe image seen by the viewer would appear to be three-dimensional. Thereis one important proviso, namely that as the viewer moves the head fromside to side, the viewer will see different views of thethree-dimensional image, but there will be gaps between each view wherethe viewer can see nothing because the field of view of eachtwo-dimensional image in the system so far described is narrow becauseeach image starts from an effective point source in the form of thesmall lenses 3. A solution to this is to add a diffuser 8 as shown inFIG. 4, comprising a grating or screen of lenslets which expands thefield of view of each two-dimensional image so that there are no gapsbetween adjacent views.

Diffusers, like lenses, suffer from aberrations in the sense that raystravelling from different angles to the plane of the diffuser arediffused by slightly different amounts. This would mean that lightcollimated at different angles by a central lens, depending on how farfrom the propagation axis the individual projector was, would bediverged by different amounts. But because the circularly symmetric lenshas no aberrations, the light for each view is properly collimated, soall the rays which comprise the view are diffused by the same amount. Itis therefore possible to close the gap between each pair of adjacentviews by moving the relevant microprojectors 4 without there being anyoverlap, even at extreme angles of view.

A problem nevertheless arises in that, at extreme angles of view, therays on one side, e.g. from the ends of the curved microdisplay line, goover the edge of the sheet of foil 5 before hitting the surface wave 7,while the rays on the other side leave an increasingly large part of thesurface wave 7 unilluminated, as shown in FIG. 5. The picture isdistorted by shear, and a dark triangular gap appears at the top of eachoff-axis view. The distortion can be anticipated and corrected bydigital pre-processing of the image in a frame, store before display onthe projectors, and digital pre-processing can also be used to eliminatethe triangular gaps provided that a pair of mirrors 9 are added to thesystem, as shown in FIGS. 6a and 6 b.

The pair of mirrors 9 are placed at either side of the sheet of foil asshown in FIGS. 6a and 6 b so that the rays going over the edge of thesheet of foil 5 and therefore “mussing” the sheet so that it cannotreflect them are reflected back. These reflected rays will now becomepart of the opposite view to that formed by unreflected rays, but indoing so the reflected rays fill in the gap left in the opposite view bythe rays in that opposite view leaving part of the surface wave 7,unilluminated. It is then a matter merely of swapping pixels in theframe store to ensure that the right pixels end up on the right positionon the screen.

Further embodiments of the invention will now be described which usediffraction gratings.

If collimated light is injected into a slab waveguide and a weakdiffraction grating is embossed on one surface of the slab waveguidethen the grating will diffract some of the light out of the waveguide.The direction in which the diffracted light leaves the waveguide will bedetermined by the initial direction of the injected light, so that bymodulating the intensity of light collimated into each of severaldirections at the input to the waveguide, one can control the intensityof light being diffracted out by the grating, and this can be used toproject an image.

FIG. 7 shows how light is injected into a slab waveguide 10 from awide-field-of-view projection display comprising a circularly symmetriclens 1 and an array of light emitters 2. Light from each pixel of thearray of light emitters 2 is collimated by the circularly symmetric lens1 in a particular direction, and this beam is coupled into the slabwave-guide 10 and diffracted out from all of one surface of the slabwaveguide 10 by the weak diffraction grating 11 so as to causecollimated light to travel in a particular direction. Other pixels ofthe array of light emitters 2 cause light to be diffracted by the weakdiffraction grating 11 in other directions, and the result is theprojection of a 2D image from a flat panel.

In general, a three-dimensional display can be created by placing afast-switching liquid-crystal display 12 over a large projectiondisplay, and FIG. 8 illustrates how this principle is applied to awide-field-of-view flat-panel three-dimensional display, by placing afast-switching liquid-crystal display 12 over the slab 10. The image canbe either autostereoscopic, in which case pixels in the array of lightemitters 2 should abut, or holographic, in which case pixels in thearray of light emitters 2 should be point sources. The only differencein practice is that holographic systems need pixels small enough to giverise to diffractive effects.

At extreme angles light from the wide field of view projection displayin FIG. 7 may miss the slab waveguide 10 as in FIG. 5. FIG. 9 shows howa pair of mirrors 9 and a one-dimensional retroreflector 13 can be usedboth to fold the optical system, so that the slab waveguide 10 can be ontop of the lens 1, and to ensure that light is injected into all of theslab waveguide 10 even at wide fields of view. Light which wouldotherwise leave the system is reflected by one of the mirrors 9 so as toland on a one-dimensional retroreflector 13, then on an angled mirror14. The planes, of the one-dimensional retroreflector 13 and angledmirror 14 are positioned at right angles to each other and at 45° to theplane of the lens, so that light is returned in the plane of the slabwave-guide 10, and the prisms of the retroreflector are runperpendicular to the long axis of the retroreflector so that lightreturns back along the same path in the plane of the slab waveguide 10as that on which the light travelled out in the plane of the wide fieldof view projector. The retroreflected light will hit the same one of thepair of mirrors 9 which it hit on its outward journey, and therefore bedirected into the slab waveguide 10 at a congruent position anddirection to that at which it left the circularly symmetric lens of thewide-field-of-view projector.

Fast-switching liquid-crystal displays can be more convenientlymanufactured if they work in reflection rather than (as in the aboveexample) transmission. This permits for example the use of thick metalwires on the back of the display which switch quickly because they arehighly conductive, but are opaque. It also permits the use of lightvalves sometimes known as optically addressable spatial lightmodulators. FIGS. 10 and 14 show shows to synthesize awide-field-of-view three-dimensional image on a light valve Light from awide-field-of-view projection display is injected into the side of aslab waveguide 10, and the slab waveguide 10 incorporates a weak grating11 but the grating 11 is blazed and volumetric so as to eject light onlytowards the front surface of the fast-switching liquid-crystal display12. Such a grating can for example be made by gluing two sheets of 3MImage Directing Film IDF II face to face with a transparent glue of aslightly different refractive index to the film. Light reflected off thefast-switching liquid-crystal display 12 travels back through the slabwaveguide 10 and on to the viewer with only minimal disruption from thegrating 11 because the grating 11 is weak. The display 12 is opticallyswitched by a projector 20. The lens 1 in FIG. 14 is a monocentric lensmade of a generally circular transparent disc the thickness of whichvaries by radius and which is adapted to receive light at the edge.

Referring to the diagram of FIG. 3, the sheet of foil 5 and surface wave7 or other ejection means can be used to convert any one-line-highthree-dimensional display into a full flat-panel three-dimensionaldisplay, and the circularly symmetric lens 1 can be used to expand thefield of view of most three-dimensional display concepts. FIG. 11 showsfor example how a holographic three-dimensional display with a widefield of view can be made by arranging that the array of light emitters2 in the focal plane of the circularly symmetric lens 1 is formed by aseries of unmodulated point sources, and that this combination is usedto illuminate a one-dimensional liquid-crystal display 15. The field ofview of a hologram screened on such a liquid-crystal display 15 isdetermined by the size of its pixels, but a hologram with a wide fieldof view can be time-multiplexed by illuminating each of the pointsources in the array of light emitters 2 and simultaneously altering thehologram on the one-dimensional liquid-crystal display 15 within thetime taken for the surface wave 7 to move the width of a single line.Wide fields of view are possible with such a display because the minimalaberrations of the circularly symmetric lens 1 allow the constituentholograms to be time-multiplexed without gaps or overlap. The singlelong LCD 15 is more difficult to make than the small microdisplays 4 ofthe previous embodiments but it is easier to make the pixels necessaryfor a hologram.

If the same set-up is intended to display three-dimensional images usingautostereoscopic rather than holographic pixellation, one line of oneview is shown at a time on the liquid-crystal display 15. The equivalentline of other views can be time-multiplexed without gaps or overlap; tothis end the array of light emitters 2 must now comprise sources oflight which abut without gaps. Also a diffuser should be used, as withprevious embodiments.

An important advantage of using the circularly symmetric lens 1 in theflat-panel concepts so far described is that with proper design it canbe stamped out of plastic in a single quick action. However, circularlysymmetric lenses can be made using bulk optics and the concept extendedto bulk optic three-dimensional displays if required.

Demand also exists for the field of view of head-mounted displays to beexpanded, and this could be done by providing a curved array of lightemitters in the focal plane of a bulk optic circularly symmetric lens,as shown in FIG. 12.

A variant of the “mechanical” method of line-by-line extraction of theimage is shown in FIG. 13. Here the output from the lens 1 is directedat a spinning right-hexagonal prism with its axis normal to the mainpropagation direction. The speed of rotation of the prism issynchronized with the line display on the projectors 4, so that when theprojector has run through all the horizontal lines of the image theprism has rotated by one sixth of a revolution and is ready to reflectthe next line towards the top of the screen once more. A lenticulardiffuser plate 18 spreads the image in the vertical direction, and aprecision azimuthal diffuser (not shown) is provided for moving theimages from the different projectors.

In general, and where appropriate, all the features of the embodimentsdescribed can be used in any desired combination.

What is claimed is:
 1. A flat-panel wide-field-of-view projectiondisplay comprising a disc-shaped, circularly symmetric lens collimatinglight from points in a focal circumference around the disc, and an arrayof light emitters positioned along the focal circumference of thecircularly symmetric lens so that light rays from each of the lightemitters are substantially collimated by the lens in the plane of thelens in a different direction from its neighboring light emitters andpass through the lens as a beam; a light modulator for modulating therays; and a ray-diverting means upon which the collimated light impingesand which ejects said light out of the plane of the lens and towards aviewer.
 2. A projection display according to claim 1, in which theray-diverting means comprises a flat panel of material.
 3. A projectiondisplay according to claim 2, further comprising line-selecting meansassociated with the panel for selecting one line at a time of an imagefrom the array of light emitters so as to display that line.
 4. Aprojection display according to claim 3, in which the ejection of thecollimated light out of the plane is by deflection of the rays from thepanel at the selected line.
 5. A projection display according to claim3, in which the ray-diverting means includes a rotating prismaticreflector (20) and the selecting means includes means for synchronizingthe rotary position of the reflector with the modulation of the light.6. A projection display according to claim 4, in which the panelincludes a reflective sheet (5) and the selecting means is a transducer(6) for producing a localized, linear, acoustic or surface wave in thesheet, the presence of the wave at a given position causing reflectionof the ray.
 7. A projection display according to claim 4, in which thepanel is a waveguide (10) into which light from the lens is injected. 8.A projection display according to claim 7, in which the ejection means(11) is a diffraction grating, which causes collimated light to travelin a particular direction.
 9. A projection display according to claim 7in which the line-selecting means comprises a layer of strip on thepanel or at any other position in the collimated beam of light, which isswitchably reflective or transparent; the means for selecting theposition at which the rays are ejected being adapted to change the stateof the switchable layer.
 10. A projection display according to claim 1in which each light emitter includes a microdisplay (4) acting as thelight modulator.
 11. A projection display according to claim 10, inwhich each light emitter comprises a microdisplay and an individual lens(3), arranged so that the microdisplay emits light towards theindividual lens; each individual lens being positioned on the focalcircumference of the circularly symmetric, lens (1).
 12. A projectiondisplay according to claim 11, in which each individual lens iscylindrical and separated from the microdisplay by its focal distance.13. A projection display according to claim 10, in which neighboringmicrodisplays each project a complete one-dimensional image, the imagesdiffering only in the angle view or phase.
 14. A projection displayaccording to claim 10, in which a frame store is provided for eachmicrodisplay to store successive images of a moving display.
 15. Aprojection display according to claim 1, in which the light emitters arepoint sources, used to display a hologram, or abutting sources, used todisplay an auto-stereoscopic view.
 16. A projection display according toclaim 1, and further including a diffuser (8) positioned after theray-diverting means in order to narrow the gaps between the beams fromadjacent light emitters.
 17. A projection display according to claim 2,which the light sources are unmodulated and the light modulator is inthe form of a switchable strip provided in the path of the collimatedrays, between the circularly symmetric lens and the panel, in order tomodulate the collimated light.
 18. A projection display according toclaim 2, further including a reflector (9), provided to at least oneside of the panel (10) to reflect an outer portion of the image thatmisses the panel back towards the panel.
 19. A projection displayaccording to claim 18, further including image-processing means adaptedto ensure that the reflected pixels display the correct part of theimage, taking into account the reflection.
 20. A projection displayaccording to claim 2, arranged with the circularly symmetric lens (1)and panel (5, 10) in substantially the same plane, preferably the planein which the light is emitted from the light emitters.
 21. A projectiondisplay according to claim 2, in which the planes in which the panel andlens are formed are adjacent and parallel, folding means being providedto fold the optical system so that rays emitted from the edge of thelens are directed into the panel.
 22. A monocentric lens comprising agenerally circular transparent disc defining two opposite faces joinedby a cicumferential edge and whose thickness varies with radius in sucha way that light can be injected into, the edge of the disc, at an angleslightly off the normal to the axis of the disc, and be totallyinternally reflected off the faces of the disc, emerging as a collimatedbeam from the set edge at location remote from the point of entry.